Preservation of the environment is increasingly being recognized as of vital importance to society if its quality of life is to avoid destructive impairment. A vital part of this effort involves the treatment of human waste from an ever-increasing population, particularly in areas of high population density. Typically, the large scale processing of human sewage involves an initial primary treatment to remove solid wastes, followed by biological, or secondary treatment of the residual sewage. In course of the biological treatment, an oxidative process, pathenogenic contamination is reduced by controlling the conditions within the treatment area to favor the propagation of microorganisms that feed on, and thus destroy the sewage. The pathogens present are killed in the process, and the biological oxygen demand of the residuum is lowered to a point at which release of the treated material back into the environment results in minimal or no threat to the public health.
Typically, secondary treatment is accomplished by bubbling or diffusing air upward from the bottom of large aeration tanks in which the sewage is confined. The treatment tanks are long, deep, e.g., in the order 15 feet, tanks equipped for air sparge into their liquid contents from multiple air outlets located along their bottom.
As will be readily appreciated, the efficiency of the oxidative process depends to an important extent upon the degree of access of air to the aerobic organisms that populate the tank. In turn, such access is directly influenced by the nature and distribution of the bubbles traveling upward in the tank, being favored by many small uniformly dispersed bubbles providing a large surface area. Porous ceramic diffusers are well suited to the generation of such bubbles, typically 2 to 3 millimeters in diameter, and such fine bubble diffusers have heretofore enjoyed widespread use in the industry.
In the past, ceramic diffusers employed for this purpose have taken various forms. For example, the diffusers originally took the form of flat ceramic plates fastened to concrete "boxes" spaced along the bottom of the treatment tanks fed with air from manifold pipes located beneath the tanks. The fact that such systems were difficult to maintain and desirably modify, however, resulted in the development of alternative systems, for instance, ceramic diffuser tubes supplied with air fed from a manifold pipe system positioned along the tank's bottom. The diffusion of a gas through a cylindrical surface tends to result in the generation of non-uniform sized bubbles, however, due in part to the tendency of some of the bubbles generated on vertical surfaces to coalesce into bubbles of larger, varying sizes.
Circular air permeable diffuser discs of the type described in U.S. Pat. No. 4,046,845 have also been used, and while these are generally satisfactory, the amount of the air-filled space, or plenum, located immediately below the disc which is necessary to resupply air lost across the face of the diffusion surface as a consequence of bubble formation is dependant in such designs upon the geometry of the structural assembly upon which the diffusers are mounted. Consequently, a certain inflexibility is inherent in such systems, insofar as varying the amount of air that can be supplied to the diffusers through their associated plenums.
A further design is that comprising a diffuser cap or dome-shaped hollow enclosure contemplated by U.S. Pat. No. 3,532,272. The device there described involves a covered, cylindrically shaped enclosure, sealed around its lower perimeter to a manifold pipe from which it is supplied with air. While such domes inherently provide their own plenum structures, a significant drawback to their use has been the non-uniformity in both the size and distribution of the bubbles formed by use of the devices.